As a result of the study of machine dynamics, numerous vibration-damping systems have been developed for reducing undesirable vibration modes that are generated when operating a machine. In order for a machine to transform or transfer energy, the machine typically has a number of fixed and moving bodies interposed between a source of power and an area where work is to be done. In operation, the bodies interact or cooperate one with the other. As an example, electric motors transform electrical energy into mechanical energy.
One form of machine that produces vibrations is a Stirling engine. A Stirling engine converts heat into reciprocating piston motion within a thermodynamic gas environment, wherein the thermodynamic gas works on the piston to create mechanical power. Such devices produce vibration when operating, principally along a single axis. Similarly, a Stirling cryogenic cooler converts electrical energy into reciprocating piston motion that operates on a thermodynamic gas via a reciprocating displacer to produce a cool region.
With nearly all types of linear reciprocating thermodynamic machines such as Stirling engines, vibration is caused by operation of the machine. For many machines, the vibration involves some form of reciprocating motion within the machine. It is frequently desirable to eliminate vibration that is created during operation of a machine. Many devices have been created for reducing, or eliminating, machine vibration.
One area where balance systems are in need of significant improvements is the field of linear motion machines, such as free piston Stirling machines. One exemplary free piston Stirling machine is a free piston Stirling cycle engine. A typical free piston Stirling engine contains a single displacer and a single power piston that cooperate in fluid communication via a thermodynamic working gas. Such an engine construction can be resolved into a machine vibration problem that principally has a one-dimensional vibration component. Such machines have relatively simple controls, but are inherently unbalanced. The reciprocating masses cooperate through the working gas, transmitting alternating forces while within a sealed vessel. Typically, operation of such a Stirling machine can produce large unbalanced dynamic vibration forces that require use of a large mounting structure to absorb forces produced during operation. Alternatively, sophisticated suspension arrangements are required to isolate the machine from its mounting structure. However, these systems frequently prove too complex and heavy where it is necessary that the system be portable and lightweight. For example, use of these devices for space exploration and remote site usage usually necessitates that the devices be constructed to have a minimized total weight.
One technique for reducing vibrations on Stirling cycle engines has been to incorporate a passive balance system, such as that disclosed in U.S. Pat. No. 5,895,033, and assigned to Stirling Technology Company, of Kennewick, Wash. Such passive balance system comprises flat spiral springs that support a counterbalance mass for axial movement relative to a housing of a vibrating machine, such as a Stirling cycle machine. The counterbalance mass is used to counteract a moving mass within the Stirling cycle machine. Spring rates for the passive balance system are chosen so as to set a natural frequency of oscillation for the passive balance system, wherein the frequency is near the operating frequency of the Stirling cycle machine. Accordingly, the passive counterbalance mass will be near resonance for the Stirling cycle machine. As the counterbalance mass oscillates, it cycles energy flow within the system between working gas and oscillating components (machine and passive balance system), and reduces the vibration forces created by the system over that which would otherwise be created solely by the Stirling cycle machine. In this manner, passive balance systems have been applied to Stirling cycle converters and coolers with a relative amount of success. However, such passive balance systems can only cancel vibration forces at a single frequency. Accordingly, passive balance systems are often only effective at reducing transmitted vibration, and typically cannot fully remove all the vibration forces, particularly for cases where a sensitive instrument is associated with the Stirling cycle machine.
For example, one application calls for use of a Stirling energy converter, in the form of a Stirling engine, to provide power for an electrical deep-space device on a spacecraft or a satellite that is orbiting the earth. Typically, either a solar collector or a nuclear heat source is used to drive the Stirling engine. The Stirling engine is mounted to a sub-structure of the satellite. However, if the Stirling cycle engine is rigidly coupled to the satellite structure, vibration forces will transfer through the coupling structure to the satellite. In some applications, such as a satellite application, transferred vibration forces will result in motion of the satellite. This resulting motion can be unacceptable for sub-systems of the satellite, such as sensitive optical equipment that may be mounted on the satellite.
Therefore, there is a need to provide an improved balance system for use with vibrating machines which provides a needed counterbalance mass that is more effective, is more adaptively controlled, and still has a relatively small overall mass. Furthermore, there is a need to provide such a counterbalance mass in a manner which can be easily tuned to accommodate specific operating frequencies of a linear motion machine, such as a Stirling cycle machine.
The present invention arose from an effort to develop an active balance system that is relatively low in cost, is relatively lightweight for a particularly sized counterbalance mass, can be implemented on a pair of opposed Stirling cycle machines or on a single machine, has vibration characteristics that can be easily tuned to a particular machine operating speed by controlling movement of the counterbalance mass relative to a Stirling cycle machine, and can be mounted with relative ease onto an existing machine along a desired line of vibration to be counterbalanced in one of several manners.